A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY. David Allan Saddoris

Transcription

1 Hydrodynamic Separator Sediment Washout Testing A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY David Allan Saddoris IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE Omid Mohseni, John Gulliver July 2010

2 David Allan Saddoris 2010

3 Acknowledgements The work reported herein was funded by the Minnesota Department of Transportation and the Minnesota Local Roads Research Board. Mike Eastling from the City of Richfield and Shirlee Sherkow from the Minnesota Department of Transportation were the technical and administrative liaisons, respectively. The members of the Technical Advisory Panel for the study, Scott Carlstrom, Mike Eastling, Jack Frost, Marilyn Jordahl-Larson, Susan McDermott, and Shirlee Sherkow, deserve recognition for their valuable input and feedback throughout the project. I would like to thank David Olson, Public Works Superintendent from the City of New Brighton, MN for allowing access and providing support during field testing on the Environment21 device. I would also like to acknowledge and thank Rinker Materials and Hydro International for providing the internal components for the Stormceptor STC1200 and the 6-ft Downstream Defender, respectively, and Environment21 for allowing testing of their V2B1 device for this project. I would like to express my sincere thanks to Kurtis McIntire, Brian Bell, Pat Brockamp, Teigan Gulliver, Matthew Lueker, Ben Plante, Adam Howard, Andrew Fyten, Mike Plante, Ben Erickson, Craig Hill, Sara Johnson, Andrew Sander, Greg DeGroot, Luke Schmidt, Ann Dienhart, Brian Sweezy and Seth Weeks from St. Anthony Falls Laboratory for helping with the experimental setup, field tests, data collection, data analysis and report preparation. Most importantly, I would like to thank Omid Mohseni and John Gulliver for providing significant expertise and guidance throughout this project. i

4 Abstract Hydrodynamic separators are widely used in urban areas for removal of suspended sediments and floatables from stormwater due to limited land availability for the installation of above ground stormwater best management practices (BMPs). Hydrodynamic separators are often sized based on relatively frequent storm events. However, during less frequent storm events, device design treatment rates are exceeded and previously captured sediments can be washed out of the devices. To study the potential for scour and washout of previously deposited sediments in the sumps of hydrodynamic separators under high flow conditions during infrequent storm events, sediment washout testing methods using mass balance were developed for both controlled field testing and laboratory testing. The developed testing methods were utilized to conduct sediment washout testing on three full scale hydrodynamic separators: 1) an Environment21 V2B1 Model 4, 2) a STC1200 Stormceptor and 3) a 6-ft Downstream Defender. All three devices were tested in the laboratory. The Environment21 V2B1 device was also tested in the field. In addition to full scale testing, two scale models were constructed and tested in the laboratory. Previous work by others on describing sediment washout in standard stormwater sumps was applied to data obtained from the sediment washout tests to develop sediment washout functions which incorporate non-dimensional parameters. Washout functions were developed for each of the devices tested in the laboratory and the field, as well as for a modified ecostorm device previously tested by others. The washout functions that were developed can be incorporated, along with removal efficiency functions, into continuous urban runoff models to predict maintenance schedules for hydrodynamic separator devices. ii

5 Table of Contents Acknowledgements... i Abstract... ii Table of Contents... iii List of Tables... v List of Figures... vi 1. Introduction Hydrodynamic Separator Overview Significance of Sediments in Stormwater Previous Studies Existing Testing Protocols Methods Sediment Washout Testing Procedure Field Testing General Field Testing Methods Characteristics of Suitable Field Testing Sites Limitations of Field Testing Laboratory Testing General Laboratory Testing Methods Advantages of Laboratory Testing Disadvantages of Laboratory Testing Methods for Sediment Measurement Sediment Measurements using Sediment Height Sediment Measurement using Weight Testing and Results Full Scale Device Testing Environment21 V2B1 - Field Testing Environment21 V2B1 - Laboratory Testing Stormceptor - Laboratory Testing Downstream Defender - Laboratory Testing ecostorm Laboratory Testing Scale Model Testing Swirl Flow Scale Model Idealized Swirl Flow Scale Model Sediment Washout Functions Sediment Washout Function Full Scale Devices Environment21 V2B Stormceptor Downstream Defender ecostorm Comparison of Full Scale Devices Scale Models Swirl Flow Scale Model Idealized Swirl Flow Scale Model...45 iii

6 4.3.3 Comparison of Scale Models Discussion Application of Results Summary and Conclusions Bibliography...52 iv

7 List of Tables Table 3-1: Stormceptor Model STC 1200 Sediment Washout Testing Results v

8 List of Figures Figure 2-1: Weight of Downstream Defender during Sediment Washout Test# Figure 3-1: Plan and Section of V2B1 Model 4, New Brighton, MN (Environment21, 2005) Figure 3-2: Environment21 V2B1 Model 4 Field Testing Results Figure 3-3: Schematic of Environment21 V2B1 Model 4 used in Laboratory Testing Figure 3-4: Environment21 V2B1 Model 4 - Laboratory Testing Results Figure 3-5: Environment21 V2B1 Model 4 Sediment Washout Testing Results Figure 3-6: Drawing of Stormceptor STC 1200 (Rinker Materials, 2009A) Figure 3-7: Drawing of 6-ft Downstream Defender Figure 3-8: 6 ft Downstream Defender Sediment Washout Testing Results Figure 3-9: 6 ft Downstream Defender Sediment Washout Testing with initial Sediment Deposit of <75% of Recommended Capacity Figure 3-10: Schematic of Modified Model 3 ecostorm (Mohseni and Fyten, 2008) Figure 3-11: Modified Model 3 ecostorm Sediment Washout Testing Results Figure 3-12: Drawing of the Swirl Flow Scale Model Figure 3-13: Swirl Flow Scale Model - Sediment Washout Testing Results Figure 3-14: Drawing of the Idealized Swirl Flow Scale Model Figure 3-15: Idealized Swirl Flow Scale Model Sediment Washout Testing Results Figure 4-1: Environment21 V2B1 Sediment Washout Functions Figure 4-2: Downstream Defender Sediment Washout Functions Figure 4-3: Downstream Defender Sediment Washout Function for an initial Sediment Deposit of <75% of Recommended Capacity Figure 4-4: Modified ecostorm Sediment Washout Function Figure 4-5: Sediment Washout Functions for Full Scale Devices Figure 4-6: Swirl Flow Scale Model - Sediment Washout Function Figure 4-7: Idealized Swirl Flow Scale Model - Sediment Washout Function Figure 4-8: Sediment Washout Functions for Scale Models vi

9 1. Introduction 1.1. Hydrodynamic Separator Overview Hydrodynamic separators are used as stormwater best management practices (BMPs) in urban areas for removing contaminants from stormwater. These underground devices are attractive in areas where land is at a premium because of their small footprint. Hydrodynamic separators are flow-through devices used as pre-treatment in a multi-bmp treatment train or as stand-alone BMPs. Water either enters these devices tangentially, thus creating a swirl, or plunges into the main sump. Hydrodynamic separators may be single or multiple sump devices. They have no moving parts and rely on flowing water as their source of energy, so they require no power. Hydrodynamic separators principally function as enhanced settling devices over a small space and commonly include a mechanism for capturing hydrocarbon products (e.g. oil) and gross solids. Consequently, they are most effective at removing heavy particulates and floatables from stormwater (US EPA, 1999), and to the extent that they are bound to larger sediments, nutrients and heavy metals. Hydrodynamic separators are less effective at removing fine particulates (US EPA, 1999) and cannot remove dissolved compounds. There are two important criteria to consider when determining the overall performance of hydrodynamic separators: 1) their efficiency at removing contaminants under treatment flow conditions and 2) their ability to retain accumulated sediments under high flow conditions. Hydrodynamic separators are sized based on the runoff from the drainage basins they serve. As most rainfall events result in discharges less than the maximum design treatment rates (MDTR) for the installed devices, removal efficiency under treatment rates is an important characteristic for assessing the performance of these devices. However, during less frequent storm events, MDTRs are exceeded, and previously captured sediment can be subject to scouring, resuspension and washout from these devices. Historically, monitoring programs have been used to assess the performance of hydrodynamic separators. Monitoring offers the advantage of assessing the performance of BMPs under a wide range of actual hydraulic and pollutant loading conditions for a given drainage basin (WILSON ET AL., 2007 AND 2009). However, monitoring is limited by the 1

10 accuracy of sample collection strategies (DEGROOT AND GULLIVER, 2009) as well as the magnitude and frequency of storm events. In addition, due to numerous uncontrolled variables in actual runoff events, it is difficult to use the results of a monitoring study to estimate a device s performance under different flow and sediment particle size conditions. As a result, new protocols for testing the performance and sediment washout of hydrodynamic separators utilizing controlled field and laboratory testing need to be developed. Carlson et al. (2006) and Wilson et al. (2007 AND 2009) have developed laboratory and field testing methods to assess removal efficiency of these devices. This thesis presents field and laboratory testing methods that can be used to assess sediment washout in these devices, as well as application of the testing methods to assess sediment washout in three commercial devices and two scale models. The effectiveness of hydrodynamic separators at capturing and retaining floatables was not studied and is outside the scope of this thesis Significance of Sediments in Stormwater Sediments in stormwater can facilitate the transport of pollutants, and are typically considered as pollutants themselves. A number of pollutants, including some nutrients and heavy metals, can bind to sediments. As sediments are transported by stormwater, any attached pollutants are also transported. The pollutants may later be released from the sediments into the water body, triggered by changing temperature, oxygen, ph, etc. The released pollutants may then be available to impact organisms and subsequently the ecosystem of receiving water bodies. In addition to the pollutants that are bound to sediments, the sediments themselves can act as pollutants. Sediments of certain sizes, compositions, concentrations and quantities can negatively impact receiving water bodies. Fine sediments increase turbidity in rivers and lakes and thus reduce light penetration, impacting aquatic habitat. Sediments can also cover sensitive fish spawning areas and interfere with fish gill function. In sufficient quantities, sediments can also fill water bodies, impeding navigation and reducing waters available for aquatic species. 2

11 1.3. Previous Studies A number of studies have been conducted to determine the effectiveness of hydrodynamic separators at removing sediments from stormwater under design water flow conditions. However, research on the washout of sediments under high water flow conditions has been limited. Avila and Pitt (2008 AND 2009) pre-loaded a full scale physical model of a catchbasin sump with solid particles and collected effluent samples to determine sediment washout. The model had free-falling plunging flows into the sump. The studies also included velocity measurements in the physical model, and development and calibration of a 3-dimensional (3- D) computational fluid dynamics (CFD) model. They found a strong dependence of washout on discharge and overlaying water level. They also found that geometry of the inlet affects the maximum velocities in the sump. Kim et al. (2007) tested sediment washout in a preloaded hydrodynamic separator using effluent sampling to determine sediment washout, and utilized the results to calibrate a CFD model. The model was pre-loaded to 50% and 100% of sump capacity, and discharges of 100% and 125% of MDTR were utilized for testing. They found that sediment height in the sump and discharge both have a significant impact on effluent concentration. Howard et al. (2010) utilized sediment washout testing procedures as generally described in this paper to test sediment washout in standard sumps in the laboratory. Mohseni and Fyten (2008) also utilized this methodology to test sediment washout in an ecostorm full scale model, with the results of this work discussed and analyzed in this thesis Existing Testing Protocols Currently, two testing protocols provide methods to assess the potential for washout of previously deposited sediments in hydrodynamic separators (WDC AND WDNR 2007; NJDEP 2009). In both methods, the sump shall be preloaded with sediments with specific particle size distributions. The distributions vary from large sand particles to silt and clay size particles. In both methods, scour potential is assessed by sampling the effluent. However, sampling of sand and coarse silt particles can result in errors that may be unacceptable (DEGROOT AND GULLIVER, 2009). In addition, assessing the scour potential of a sediment 3

12 gradation with a wide range of particle sizes will not illustrate how these devices function under conditions with different particle sizes and particle densities in stormwater runoff. 4

13 2. Methods 2.1. Sediment Washout Testing Procedure To overcome the limitations of sampling protocols that are frequently utilized for sediment washout testing, a new methodology utilizing a mass balance has been developed at the St. Anthony Falls Laboratory (SAFL) to assess sediment resuspension and washout in hydrodynamic separators under discharges exceeding their design MDTRs. The test procedure is as follows: 1) Drain and clean the device 2) Pre-load the device sump with sediments of known particle sizes 3) Measure the amount of sediments in the device 4) Flow water through the device for a set time duration at rates at or above the device Maximum Design Treatment Rate (MDTR) 5) Measure the amount of sediments remaining in the device 6) Determine the amount of sediments washed out via mass balance This procedure has been applied in the testing of four full scale commercial devices: 1) an Environment21 V2B1 Model 4 in the field and in the laboratory by the author, 2) a Stormceptor STC 1200 in the laboratory by the author, 3) a 6-ft diameter Downstream Defender in the laboratory by the author and 4) an ecostorm Model 3 by Mohseni and Fyten (2008). In addition, this procedure has been applied in the testing of two scale model devices by the author Field Testing General Field Testing Methods Hydrodynamic separators can be field tested for sediment washout under controlled conditions using in service devices, the aforementioned procedure and mass balance methodologies as described in this thesis and in Saddoris et al. (2010). First, the device is cleaned and preloaded with sediment and the sediment measured using the height measurement methodologies described in Section Next, clean water is provided to the device at a set discharge for the design duration of the test. The test discharge can range 5

14 from the MDTR to the maximum hydraulic capacity of the device, which can be several multiples of the MDTR. At the conclusion of the test the water supply is shut off, sediments in suspension are allowed to settle, and water in the device is pumped out. The height of the sediment remaining in the device is measured, and the average effluent concentration is calculated based on sediment washout, discharge, and duration of the test. A significant amount of water must be available at a constant discharge for the duration of sediment washout tests, which can last longer than 60 minutes. Fire hydrants were identified as the only suitable source of water for washout testing. Water discharge can be measured by installing a weir and pressure transducer in the sewer piping, either before or after the hydrodynamic separator. Laboratory calibration is required to determine discharge as a function of water height for a given pipe size and weir design. Discharges can be monitored and recorded throughout testing on a computer. Sediment preloading should be to a sufficient depth that washout to the floor of the device during the testing will be avoided, limiting error in sediment height measurement. Sediment loading of at least six inches is likely necessary to achieve these goals, which corresponds to a sediment charge of 1,000 pounds or more for most field devices of a size suitable for physical access for testing. It is not practical to sieve this quantity of sediments, particularly to perform multiple tests with sediment replacement between each test. Therefore, commercial sand gradations with narrow particle size distributions should be utilized for testing Characteristics of Suitable Field Testing Sites Installed hydrodynamic separators suitable for field testing must meet several criteria: sufficient water availability, ability to isolate the device, adequate device size, convenient site, and accessible and safe site. 1) Water supply with a constant discharge must be available at the site. Fire hydrants are the only suitable source of water identified for sediment washout testing in the field. 6

15 2) The hydrodynamic separator should not be subject to flows from other sources. Flows of water into the device from uncontrolled sources prevent testing under steady state conditions and complicate draining the device to take sediment measurements. 3) The device must be of an adequate size. The device must be large enough to allow physical access of the test crew for sediment measurements and cleaning, but must be small enough that local fire hydrant(s) can provide sufficient water flows up to several times the MDTR. 4) The site must be convenient. Mobilization, setup, testing, cleanup and demobilization are required to complete each field test. 5) The site must be accessible and safe. Hydrodynamic separators are commonly installed adjacent to roadways. Sites near heavy traffic introduce hazards to workers and testing can interfere with the flow of traffic Limitations of Field Testing There are a number of limitations that make controlled sediment washout field testing in installed devices challenging and inefficient: 1) Suitable installed devices are difficult to find; most devices do not meet the criteria provided above for testing suitability. As the devices must be reasonably large for personnel access, high water discharges must be provided, but water flows from adjacent fire hydrants are commonly insufficient for washout testing. 2) Securing the site and sediments is required whenever the site is not attended; setting up and breaking down water supply and measurement and confined space entry equipment, and transportation to and from the site, require significant time for test personnel. 3) Testing is dependent on weather. Testing cannot be scheduled when rain events are forecast, as uncontrolled water flows into the device prevent testing, and even a small amount of rain can ruin a test in progress and destroy a day s field work. 4) Hydrodynamic separators are commonly confined spaces, so appropriate equipment, personnel and training must be provided for testing. Testing costs are high due to the required support. 7

16 2.3. Laboratory Testing Due to the inherent limitations and inefficiencies associated with controlled field testing of hydrodynamic separators, methodologies were developed and implemented for laboratory mass balance sediment washout testing of full scale hydrodynamic separators General Laboratory Testing Methods Field installations of hydrodynamic separators typically consist of concrete manholes with some internal components. Large concrete manholes are heavy and difficult to move within a laboratory environment. Therefore, sumps were constructed at Saint Anthony Falls Laboratory (SAFL) by fabricating fiberglass tanks to full scale device specifications and installing internal components into the fiberglass tanks. Proprietary internal components were provided by device manufacturers. The fiberglass tanks with installed internal components were setup on steel frames, which rested on load cells for weight measurement as described in Section Piping from the SAFL main water supply channel provided sufficient water supply for testing of full scale models up to at least their maximum hydraulic capacity. Water discharge was set by adjusting a gate valve upstream of the model. Water discharge was measured and recorded during testing of an Environment21 device using a calibrated circular weir and pressure transducer in the discharge piping. Discharge was measured and recorded during testing of a Stormceptor device and a Downstream Defender device using a sharp-crested weir. Free outfall from the sump outlet pipes were directed into a rectangular channel, with a rock crib wall to create a uniform approach flow and remove surface waves before reaching the sharp-crested weir discharge. The test procedure described for field testing can also be used for sediment washout laboratory testing. First, the device is preloaded with sediment. Unlike field testing, there are two methods available for determining sediment amounts in devices: 2) height measurement methodologies as described in Section and 2) weight measurement methodologies as described in Section Next, clean water is provided to the device at a set discharge for the design duration of the test. The test discharge can range from the 8

17 MDTR to the maximum hydraulic capacity of the device, which can be several times the MDTR. At the conclusion of the test, the water supply is shut off and sediments in suspension are allowed to settle. The sediment remaining in the device is measured using height and/or weight techniques, as described in Section 2.4. The effluent concentration is calculated based on sediment washout, discharge and the duration of the test. As with field testing, substantial sediment amounts are required for full scale laboratory sediment washout testing. Commercial sand gradations with narrow particle size distributions should be utilized for testing Advantages of Laboratory Testing By laboratory testing full scale hydrodynamic separator device models, the limitations of field testing are overcome: 1) Adequate water supply for high flow conditions can be available. 2) Models can be of an adequate size for access, i.e. with safe access and good egress. 3) Testing is not dependent on weather. 4) The washout rate can be measured by weighing the device Disadvantages of Laboratory Testing There are several disadvantages to laboratory sediment washout testing using mass balance: 1) Whereas field testing utilizes existing equipment, laboratory testing requires construction of a test model. Large full scale models require significant materials and labor to construct. 2) Laboratory testing requires cooperation of the device manufacturer for utilization of their design in a test model. Field testing could be conducted in some installed devices without an agreement with the device manufacturer. 3) Although the Saint Anthony Falls Laboratory has substantial water availability due to supply from the Mississippi River at a natural waterfall, many laboratories depend on recirculating water and may not have water availability at the high discharges required for sediment washout testing. 9

18 2.4. Methods for Sediment Measurement To determine sediment washout during testing using mass balance, it is essential to accurately measure sediment at the beginning and the end of the test. The beginning and ending sediment amounts, along with information on discharge throughout the testing, can be used to calculate average effluent concentration. In addition, washout rates during testing can be calculated if accurate measurements of sediment in the device during testing are obtained. Two general approaches were used to measure the amount of sediment in the devices: 1) sediment height measurements and 2) sediment weight measurements. Sediment height measurements require the device to be drained for access to take measurements of the height of the wet deposit. Therefore, this approach can only provide total sediment change over the course of the entire run, and only average effluent concentration can be calculated. An alternative approach is to take pre and post-run sediment weight measurements using load cells as described in this section. Accurate sediment weight measurements during testing can also be obtained with load cells. Therefore, in addition to providing an average effluent concentration for the entire run, sediment weight measurement can also provide effluent concentrations during testing Sediment Measurements using Sediment Height Two methods were used to determine the height of sediment in an Environment21 V2B1 device tested in the field: 1) Utilizing a ruler measuring the depth of the sediment to the floor of the device and 2) utilizing a laser range finder measuring the distance between the top of the sediment and the ceiling of the device. Ruler Stick Method Following sediment leveling, a standard metal ruler stick was used to measure the depth of the wet sediment at 18 locations in the device. The same 18 locations were used for each sediment height measurement, and the 18 readings were averaged to obtain average sediment depths. 10

19 Laser Range Finder Method A Hilti Model PD 30 laser range finder was used to measure the distance from the top of the sediment to the ceiling of the device. This laser range finder model has a published accuracy of ±1/16 th of an inch over its entire measurement range of 660 feet. A custom bracket was fabricated, and the laser range finder was securely fastened in the bracket. The bracket incorporated a bubble level. The laser range finder was used to measure the distances between the top of the sediment and known reference points. The 28 known reference points used for the Environment21 device field testing included marked locations on the ceiling of the device, on the bottom of the inlet pipe and on a cover placed over the manhole opening. To take a reading the laser range finder bracket was placed on the top of the sediment and moved until the bracket was level and the laser beam was on target on the reference mark. Before the first washout test, readings were taken when the device was empty to determine the distances between the floor of the device and the obstruction (i.e., ceiling, bottom of inlet pipe or manhole cover) at each of the 28 reference points. The depth of sediment at each point was determined by subtracting the distance between the top of the sediment and the obstruction from the reading for the empty device at the same reference locations. Readings were taken at each of the 28 points before and after each washout test. The 28 sediment depths were averaged to obtain average sediment depths. Verification of Sediment Height Test Methods Prior to the beginning of the Environment21 V2B1 field testing, the repeatability of the laser range finder was verified by measuring the deposit height several times in a full scale Royal Environmental Systems ecostorm Model 3 device at Saint Anthony Falls Laboratory. The deposit was moved around and re-leveled between each test to simulate actual sediment washout testing. This testing demonstrated that the laser range finder method can produce repeatable results, as the difference between the high average reading and the low average reading was only 0.08 inches, or 0.7% of the average total sediment height (SADDORIS ET AL., 2010). To verify the repeatability of the laser range finder and the stick measurement techniques in the field, a test was conducted in an Environment21 V2B1 device in New Brighton, MN. For 11

20 these tests, the sediment deposit was measured several times using both the stick measurement and the laser range finder methods, with mixing and re-leveling between each test. For the laser range finder, the difference between the high reading and the low reading was 0.13 inches, or 1.9% of the average total sediment height. The ruler stick was even more repeatable in this field test, with a difference between the high and low reading of only 0.05 inches, or 0.8% of the average total sediment height. The two sediment height measurement methods also compared favorably to each other, with the average sediment depth for the three tests for the laser range finder being only 0.11 inches, or 1.7% of the total average sediment height, different than the average of the sediment depths for the three tests with the ruler stick method (SADDORIS ET AL., 2010). Both measurement techniques (the laser range finder and the ruler stick) were used to measure pre-run and post-run sediment heights for eight of fourteen sediment washout tests (described in the Section 3.1) conducted in an Environment21 device in New Brighton, MN. The results show that both sediment height measurement techniques have good agreement, as the average of the absolute values of the differences for the pre-run measurements is 0.15 inches, or 2.3% of the average total sediment height, and the post-run measurements is 0.13 inches, or 3.0% of the average total sediment height. The standard deviation for the absolute values of the differences for the pre-run measurements is 0.13 inches, and the standard deviation for the post-run measurements is 0.09 inches (SADDORIS ET AL., 2010). The verification testing demonstrates that the metal ruler stick and laser range finder sediment height measurement techniques are repeatable and compare favorably to each other within tolerances that are reasonable for full scale sediment washout testing. Test Methods Sediment loss can be calculated utilizing the change in sediment height and bulk density of the wet sediment using Equation 2.1. = (2.1) In Equation 2.1, H i and H f are the initial and final heights of the sediment deposit, A cs is the cross-sectional area of the sump, ρ d is the bulk density of wet sediment, Q is discharge, and 12

21 t is the test duration. The bulk density of wet sediment is the weight of dry sediment per volume of wetted sediment. The bulk density of the wet sediment was determined by charging a full scale device with a known weight of dry sediment, wetting the sediment, and measuring the volume of the sediment using the stick measurement technique. The bulk density was also verified in the laboratory using wetted sediment in a graduated cylinder. Discussion on Height Measurements for Sediment Washout Testing Using height measurements for sediment washout testing was found to be repeatable, and the ruler stick and laser range finder sediment height measurement techniques are both suitable for sediment measurements. As the ruler stick method is less complicated, more efficient and provides a similar level of repeatability as the laser ranger finder method, the ruler stick method is preferred for sediment washout testing. The measurement errors introduced with both the sediment height measurement techniques are largely independent of the amount of sediment measured. Therefore, test durations should be of sufficient length that significant washout occurs and measurement errors become insignificant Sediment Measurement using Weight For hydrodynamic separator testing at St. Anthony Falls Laboratory, precision strain gauge load cells were used to measure the weight of hydrodynamic separator devices and their contents. Strain gauge load cells utilize a transducer to convert force applied (i.e. weight of the device) to a measureable electric signal. As the weight in the device changes the force applied to the load cell changes, and the electrical resistance across the gauges changes in proportion to the load. Equipment for Sediment Weight Measurements During setup, the full scale hydrodynamic separator models to be tested were set on a steel frame. The steel frame was then jacked up and load cells were installed under the steel frame. Depending on the device tested, either three or four load cells were used. The load cells had load buttons installed, so the weight of the system was directly applied to a specific point on each load cell. One load cell sat on a fixed plate. The remaining load cells (two or 13

22 three depending on the device tested) sat on plates with bearings to allow movement laterally. Free lateral movement prevented the system from being constrained and prevented side loading on the load cells, which would introduce error in measurements and could damage the load cells. To measure weight with high repeatability, clean power must be provided to the load calls. Building power at 120V was supplied to an Uninterrupted Power Supply (UPS), which served to provide continuous power with level voltage and current to the load cell system. From the UPS, power was supplied to Interface Model SGA signal conditioners, which transformed and filtered the power supplied to the load cells. Each load cell was supplied conditioned low voltage power by its own dedicated Interface SGA box. In the load cells, weight applied changes the resistance across the strain gauges, causing a proportional change in output voltages. The low voltage output signals from the load cells were amplified and continuously recorded using a computer. For calibration and troubleshooting, the computer also continuously recorded the building voltage and frequency, and the voltage and current supplied to each load cell. Tovey Engineering Model FR10-5K load cells were used for full scale device washout testing. These load cells have a capacity of 5,000 lbs each, and a published non-repeatability of ±0.01% of total capacity, or ±0.5 lbs/load cell. Considering all errors (non-repeatability, amplification errors, and an over constrained system when four load cells are used), the estimated accuracy of the complete load cell system was ±0.04% of total capacity, or ±8 lbs when four Model FR10-5K load cells were used and ±6 lbs when three Model FR10-5K load cells were used (SADDORIS ET AL., 2010). Once the load cells were installed, LabView computer software was utilized to display real time information about load cell voltage input/output, weight readings, and the standard deviations of weight measurements. The water level in the sump before and after testing was accurately measured using a stilling basin and a point gauge. Additional error was introduced into the test results due to the accuracy of the water level reading. The point gauges that were utilized for SAFL testing 14

23 have a resolution of ft. The errors introduced due to water level readings are estimated to be less than 2 pounds for devices up to six feet in diameter. Test Methods The detailed procedure for hydrodynamic separator model weight measurement using load cells was described by Saddoris et al. (2010). Sediment loss can be calculated utilizing the weight change of the entire system, the density of the sediment and the density of water; average effluent concentration can then be calculated with known discharge and test duration as shown in Equation 2.2. = (2.2) In Equation 2.2, W i and W f are the initial and final weights of the device, W w is the weight change due to the difference in water level in the device, SG s is the specific gravity of the dry sediment, Q is discharge, and t is the test duration. W w can be calculated using Equation 2.3. = (2.3) where, L f is the final water height, L i is the initial water height, A cs is the cross-sectional area of the sump and ρ w is the density of water. Verification of Sediment Weight Test Method Several tests were conducted to verify the repeatability of the sediment weight test method. After a device was setup and the load cells calibrated, the first verification test was for drift. The device was partially filled with water and the weight was recorded for a period of at least 12 hours to verify that the measured weight drift was minimal. The observed drift during verification testing was less than one pound over 12 hours. The next verification test was for repeatability. This test involved recording the water level and weight of the device, adding additional water to the device until discharge occurred, and then draining the device to approximately the initial level of water and recording the weight and water level. The first repeatability test demonstrated a non-repeatability of 16 pounds. It was discovered that stresses in the system were not being relieved when the weight was removed, and the procedure was changed to include tapping the steel frame with a rubber mallet prior to taking 15

24 a final reading. After the procedure was modified, all subsequent tests had observed nonrepeatability of less than six pounds, or less than 0.1% of the total weight. Next, the accuracy of weight readings was verified by adding at least 500 lbs of water and then removing at least 500 lbs of water using buckets. The added and removed water was weighed on a scale, and the weight of water added or removed was compared to the weight readings from the load cells. The maximum disagreement between the two weight measurements was six pounds. The load cell verification testing indicated that the load cell testing method is repeatable and accurate to within approximately six pounds over the range of static weight readings anticipated for sediment washout testing. In addition, the drift tests indicate that instrument reading drift does not introduce significant error over the anticipated durations of washout tests, provided that ambient air temperature is stable 1. For four sediment washout tests in an Environment21 V2B21 device in the laboratory, both the sediment height and the sediment weight measurement methods were used. The calculated effluent concentrations in these two test methods differed from as low as 2 mg/l to as high as 28 mg/l. Discussion on Weight Measurements for Sediment Washout Testing The weight measurement method has the following uses: 1) To obtain accurate static weights before and after each test, which can be used to calculate sediment washout and average effluent concentration. 2) To observe instantaneous changes during each test. This can be used to identify any problems in the test. 3) To record dynamic weights during the tests (requires post-run analysis), which can be used to determine changes in effluent concentration during a test; i.e. whether effluent concentration is steady or time variant. Figure 2-1 shows one example of the weight of the system during a sediment washout test. In this test, at a constant discharge, a non-linear change in weight is observed, i.e. a change in the effluent concentration with time. Although there is noise in the instantaneous weight readings during the experimental runs, weight averaging over a reasonably short period of 1 Rapid changes in ambient air temperature were observed to have a significant effect on load cell weight readings. 16

25 time can be used (post-test) to calculate effluent concentrations as a function of time during the test. Figure 2-1: Weight of Downstream Defender during Sediment Washout Test#15 Sediment measurement using load cells offers several advantages over height measurements: 1) Devices can be tested that do not have accessible sumps. 2) Effluent concentrations can be measured throughout the test (as described above). 3) Tests can be completed quickly, as the sump does not need to be fully drained, the device entered, and sediment heights taken. 4) The sediment bed does not need to be disturbed by entering the sump and leveling the sediment, potentially allowing for tests that more closely replicate actual in service conditions. There are also several disadvantages with using load cells for sediment washout testing: 1) To obtain the very accurate weight measurements required for accurate effluent concentration determination, specialized equipment and expertise are necessary to setup and maintain the data acquisition system. 17

26 2) Precision load cells are very sensitive to temperature, so the temperature of the laboratory space utilized for testing must monitored, and the system must be allowed to restabilize prior to testing if the temperature of the room changes. 3) Precision load cells are sensitive to supply power conditions. Even with a UPS, filtering equipment, and voltage monitoring and compensation, due to high amplification moderate changes in supply power can influence weight readings. Therefore, the test facility needs to have a relatively clean power supply with limited influence from large electrical equipment. The measurement errors that are introduced with the sediment weight measurement technique are largely independent of the amount of sediment measured. Therefore, test durations should be of sufficient length that significant washout occurs and measurement errors become insignificant. 18

27 3. Testing and Results 3.1. Full Scale Device Testing Environment21 V2B1 - Field Testing Controlled field testing of an Environment21 V2B1 Model 4 was completed utilizing the procedure presented in Section 2.1 in the summer and fall of 2007 on an in-service device in New Brighton, MN. A drawing of this device is shown in Figure 3-1. The equipment and methods for testing this device are described in detail in Saddoris et al. (2010). Figure 3-1: Plan and Section of V2B1 Model 4, New Brighton, MN (Environment21, 2005) A total of fourteen sediment washout tests were completed under a variety of discharges, and with two sediment gradations. Six tests were conducted in the summer ( Summer Testing ), followed by six duplicate tests in the fall ( Fall Testing ). For the first twelve tests, US Silica F110 gradation was utilized. Two additional tests were conducted in the fall using 19

28 AGSCO gradation. US Silica F110 and AGSCO are commercial silica sand gradations with specific gravities of approximately 2.6. US Silica F110 has a d 50 of 120 µm, a d 15 of 80µm and a d 85 of 170µm. AGSCO has a d 50 of 200 µm, a d 15 of 120 µm and a d 85 of 280µm. Sediment was added to the device sump prior to each test so that the starting sediment depth was approximately six inches. The manufacturer s recommended sediment depth requiring cleaning is 6-12 (ENVIRONMENT21, 2009). Before and after water flow, a trowel was used to flatten and level the surface of the deposit and a 2 foot level was used to check if the deposit was truly level. After leveling, the ruler stick method and the laser range finder method were used to determine the height of the sediment in the V2B1 device 2. The sediment washout tests were conducted with water supplied from a fire hydrant. The discharge was set at the beginning of the tests and was constant during each test. The tests utilized discharges ranging from 1.7 to 4.1 cfs. The MDTR for this device is 1.4 cfs. The test durations ranged from 30 to 120 minutes. The results of the tests are shown in Figure ,400 Environment21 V2B1 - Field Testing C, Outlet Concentration, mg/l 1,200 1, Discharge, ft 3 /s F110 - Summer Testing F110 - Fall Testing AGSCO Figure 3-2: Environment21 V2B1 Model 4 Field Testing Results 2 The laser range finder method was used for all fourteen tests; the ruler stick method was also used for eight of the fourteen tests. 20

29 All reported test results for the Environment21 V2B1 are for sediment washout in the primary chamber only. At the conclusion of each test, any sediment that accumulated in the secondary chamber during the test was removed and disposed. Any sediment that accumulated in the secondary chamber was considered to be washed out from the device. Visual observations indicated that sediment accumulation in the second chamber was minimal, and never appeared to be more than a light dusting on the bottom of the sump. The low accumulation of sediment in the secondary chamber is likely due to the baffle wall design. Water in the secondary chamber is directed under the baffle wall, providing high velocities at the device floor which prevent suspended sediments from settling, and also cause scour and resuspension of any sediments that may settle at low flow conditions. By reviewing the data in Figure 3-2, it was evident that the AGSCO sand was better retained than the US Silica F110 gradation. Larger particles are less likely to be washed out, due to the higher critical shear stresses, i.e. the shear stress required to scour particles, and their higher settling velocities, which increase the likelihood that resuspended particles will resettle. The repeat tests on the F110 silica sand gradation, which were conducted later in the year, demonstrated bias towards lower washout rates. The main difference between the two test series was water temperature. The water temperature was believed to be lower for the repeat tests in the fall than for the initial tests in the summer. Even though the water temperature was monitored during the tests, the thermometer was found to be inaccurate after the fifth test, so the water temperature was unknown for the first five tests. Therefore, it became impossible to accurately assess the impact of water temperature on the results. Nevertheless, the effects of water temperature can be considered, and discussion follows. Viscosity increases as water temperature decreases. The settling rate of suspended sediments decreases as the viscosity of the fluid increases; thus, with colder water, i.e. more viscous fluid, any sediment that was resuspended by scour will stay in the water column longer. Sediments that stay longer in suspension have a higher likelihood of being washed out of the device, leading to higher effluent concentrations. However, fluids with higher viscosity dissipate more energy in the water column, so less energy should be available at the sediment bed to scour 21

30 particles when water temperatures are lower. Lower bed forces should correlate with lower resuspension of particles, which should lead to lower effluent concentrations. These two effects of viscosity (particles settling velocity and energy dissipation) are believed to partially counteract each other when considering the effect of varying water temperature on sediment washout in hydrodynamic separators Environment21 V2B1 - Laboratory Testing An Environment21 V2B1 Model 4 was tested at SAFL in December 2008 and January 2009 following the procedure described in Section 2.1. A schematic of this device is shown in Figure 3-3. The device tested in the laboratory in 2008/2009 had a smaller outlet pipe from the first chamber to the second chamber than the device tested in the field in The equipment and methods for laboratory testing of this device are described in detail in Saddoris et al. (2010). The washout was measured by monitoring the weight of the device. (Plugged for washout tests) Figure 3-3: Schematic of Environment21 V2B1 Model 4 used in Laboratory Testing Twelve tests were conducted utilizing US Silica F110 gradation, and five tests were conducted using AGSCO gradation. The tests were conducted with water supplied from the Mississippi River, which had temperatures ranging from 0 to 2 degrees Celsius during these tests. The discharge was set at the beginning of the tests and was constant during each test. The tested discharges ranged from 1.7 to 3.5 cfs. The test durations ranged from 45 to 180 minutes. The results of the testing are shown in Figure

31 800 Environment21 V2B1 - Laboratory Testing C, Outlet Concentration, mg/l Discharge, ft 3 /s F110 AGSCO Figure 3-4: Environment21 V2B1 Model 4 - Laboratory Testing Results The results of the field and laboratory testing of the Environment21 V2B1 Model 4 devices are shown on the same graph in Figure 3-5. In spite of the difference in the test methods and the connecting pipe between the two sumps, the overall washout rates obtained from the laboratory and field tests are in general agreement. 1,400 Environment21 V2B1 - All Testing C, Outlet Concentration, mg/l 1,200 1, Discharge, ft 3 /s F110 - Field Summer Testing F110 - Field Fall Testing AGSCO Field Testing F110 - Laboratory Testing AGSCO Laboratory Testing Figure 3-5: Environment21 V2B1 Model 4 Sediment Washout Testing Results 23

32 Stormceptor - Laboratory Testing A Stormceptor Model STC 1200 was tested at SAFL in December 2008 following the procedure in Section 2.1. A drawing of this device is shown in Figure 3-6. The equipment and methods for testing this device are described in detail in Saddoris et al. (2010). Figure 3-6: Drawing of Stormceptor STC 1200 (Rinker Materials, 2009A) Prior to the first test, approximately five inches of US Silica F110 gradation was charged to the sump of the Stormceptor device. Three tests were conducted using only US Silica F110, and then four tests were conducted by adding approximately three inches of US Silica Sil- Co-Sil 250 (SCS 250) on top of the F110 and allowing the SCS 250 to settle overnight, bringing the entire sediment height to approximately eight inches. US Silica SCS 250 is a commercial silica sand gradation with a specific gravity of approximately 2.6. US Silica SCS 250 has a d 50 of 45 µm, and a d 85 of about 120 µm. 24